Multiple variations on sapphire ingot growth (e.g., the Kyropoulos, Czochralski, Bagdasarov methods) involve crystallization on a sapphire seed within a crucible where molten sapphire is formed. In various growth methods an accurate knowledge of the volume of remaining molten sapphire is essential and can be ascertained via knowledge of the level or height of the fluid. For instance, since sapphire crystal is denser than the molten fluid, as the fluid crystallizes, the fluid level recedes, and the ability to monitor this changing level allows an inferred measurement of crystal growth rate.
Prior methods have used single-beam triangulation to measure the fluid level (see FIG. 1), but are difficult, if not impossible, to implement due to the requirement of a large angle (e.g., 20°) between the beam source and beam detector (e.g., MICRO-EPSILON's OPTONCDT). The chambers in which sapphire is melted can reach 2050° C., and in order to maintain such temperature, thick crucible walls are often used, allowing only a single long and narrow view corridor (or window) into the crucible. The ˜20° angle required by these methods is incompatible with such a long and narrow tube.
FIG. 1 illustrates a traditional single-beam triangulation system for remote displacement sensing. A vessel 102 contains a liquid 103 existing at a first level 104, and then at a second level 106. A single-beam triangulation-based displacement monitor 108 uses a laser source 110 to project an incident beam 112 onto the first and second surfaces 104, 106 of the liquid 103. A reflection 114 from the first level 104 is measured by a sensor 116 and the reflection 114 from the second level 104 is measures by the sensor 116. Based on the distance 120 between the positions of the reflected beams 114, 118 on the sensor 116, a distance 122 between the levels 104, 106 can be calculated. However, this system is generally inoperable in sapphire growth furnaces since displacement measurements typically are made through a narrow and elongated view corridor 202 as illustrated in FIG. 2, and the angle between the beams is typically greater than allowable through the narrow and elongated view corridor 202.
While some systems can make remote displacement measurements using smaller incident angles, and are thus compatible with the narrow and elongated view corridor 202 of a furnace 200, such systems typically do not achieve desired height resolutions for the fluid surface. These systems often use a single beam reflected off the molten sapphire fluid surface, and measure fluid height changes as a function of change in reflected beam position on a CCD. The reason such systems achieve low resolution is that a number of variables, other than movement in the elevation of the fluid level, lead to movements in the reflected beam position. Vibrations of the crucible, misalignment of the laser source, and fluctuations in the fluid surface can all cause the reflected beam to jitter across the CCD even where no change in the fluid level occurs. Furthermore, reasonably-priced CCD's are generally two-dimensional pixel arrays capable of no more than around 70 Hz refresh rates, and as a result, the rapid movements of the reflected beam are often detected as blurs rather than as ideal Gaussian circles or ellipses. All of these factors make accurate and highly resolved measurements of the fluid level difficult.
A further problem with the systems of the prior art is that the laser reflection is often overshadowed by the blackbody radiation of the over 2000° C. molten sapphire. The CCDs used in the art tend to saturate when sapphire reaches 1200° C., thus having very high signal to noise ratios long before the sapphire reaches its melting temperature.
One solution to this problem has been to use lasers operating at wavelengths where there is less blackbody intensity from the molten sapphire (e.g., blue such as MICRO-EPSILON's OPTONCDTBL). However, even when using short wavelength lasers, the CCD's tend to saturate above 1200° C.
Another attempted solution is the use of confocal displacement sensors, which use multiple frequencies of light focused at different distances to make nanometer resolution high refresh rate displacement measurements, but such devices are limited in range (e.g., MICRO-EPSILON'S CONFOCALDT has a maximum range of 24 mm). Confocal sensors are typically not applicable to molten sapphire measurements since the crucible design often requires that the sensor be around 1.5 m from the fluid surface.